We have continued our investigations in Venezuela of the potential usefulness of the rhesus rotavirus MMU18006 strain vaccine candidate. This virus is a simian rotavirus strain derived from a stool of a 3.5 month old rhesus monkey with acute diarrhea. In phase I and II double-blind studies, the vaccine was administered to 120 infants (1-10 months old). A similar number of infants received placebo. The infants were examined daily for presence of side reactions. Rectal temperature was taken twice a day by medical personnel and stool specimens were collected daily and analyzed for rotavirus shedding. Significant reactions were not observed in the infants who received the vaccine when compared to the placebo group. Serological assays performed for rotavirus antibody included complement-fixation, plaque reduction neutralization and tube neutralization. Overall, 69.3 of the vaccinated children developed a seroresponse. Phase I studies in newborn infants have also been initiated. Efficacy studies are being carried out by continuous surveillance of the 1-10 infants in the vaccine and placebo groups.